Fibre Channel (FC) is a general name for an integrated set of standards being developed by ANSI (American National Standards Institute) whose purpose is to act as a universal high-speed interface for computers and mass storage. FC is a data transfer protocol that provides a highly reliable, gigabit interconnect technology that allows concurrent communications among workstations, mainframes, servers, data storage systems and other peripherals using Small Computer Systems Interface (SCSI) and Internet Protocol (IP) protocols. FC supports multiple topologies, including a Fibre Channel Arbitrated Loop (FC-AL), which can scale to a total system bandwidth on the order of a terabit per second.
From a logical point of view, an FC-AL is a single, continuous path composed of links and nodes, wherein each node has at least one port which can act as a transmitter, receiver or both. A Fibre Channel Arbitrated Loop (FC-AL) topology can be produced by simply connecting the transmit output portion of a node's port to receive input portion of another node's port, such connection existing between 3 or more devices in a daisy-chain formation. This connection arrangement allows a circular data path or loop to be created, but poses significant problems for trouble-shooting and adding or removing devices. In order to add a new device for example, the entire loop must be downed as new links are added. If a fibre optic or copper cable breaks or a transceiver fails, all cables and connectors between all devices must be examined to identify the offending link.
Hubs resolve these problems by collapsing the loop topology into a star configuration. Instead of connecting devices directly together, each device is connected to a port on a hub. The hub completes the connection from device to device. Since all devices are connected centrally to a hub, the hub becomes the focal point of additions or moves or changes to the network.
FIG. 7 shows the internal architecture of a conventional hub with four hub ports (P0, P1, P2 and P3) to which are connected three disks (D0, D1 and D2) so that hub port P1 has no disk connected to it. The main advantage of a hub is that each hub port comprises a port bypass circuit (PBC) or Loop Relay Circuit (LRC), shown in FIG. 7 as PBC0, PBC1, PBC2 and PBC3.
These circuits enable the fibre channel arbitrated loop (FC-AL) to be opened and closed and thereby dynamically reconfigured if a device is added or removed to the FC-AL. The PBC is comprised of a 2-input MUX and a switch connecting either one of the MUX inputs to the output. The MUXs in the PBCs are connected in sequence such that the output of each MUX acts as one of the inputs for the next MUX. The loop is completed by connecting the output of the final MUX in the hub (MUX3) to the one of the inputs of the first MUX (MUX0).
For example, looking at hub port P2, it can be seen that the output of the MUX from the PBC of the preceding hub port, namely MUX1, is transmitted both to hub port P2 and to one of the inputs of MUX2. On reaching P2, the output from MUX1 is transmitted to the connected device D1. The response from D1 is transmitted back through the hub port P2 to the other input of MUX2. In use, if a hub port (in this case P2), detects that a device is connected to it, then the switch (in this case S2) in the MUX (in this case MUX2) of the hub port's PBC (in this case PBC2) directly connects the response signal from the attached device (in this case D1) to the MUX output.
Looking at hub port P1 for example, if however, a hub port detects that a device is not connected to it, or is not responding, the switch (S1) is toggled so that it directly connects the input from the preceding MUX (MUX0) to the output of its own MUX (MUX1). This acts to close the associated PBC (PBC1) and bypass the hub port (P1) thereby allowing the loop to remain intact. This prevents a failing device or connection from bringing down the entire loop.
If it is desired to add a new device, at port P1 for example, the loop opens automatically to add the new device without manual intervention, by toggling the position of the corresponding PBC switch (S1). By this, the hub allows hot plugging; the ability to add and remove devices while the loop is active. In future versions of FC-AL, before a new device is allowed to be inserted in the loop, the hub will, at a minimum, verify valid signal quality. If a device exhibits poor signal quality or inappropriate clock speed, the associated PBC switch will remain toggled to bypass the hub-port, thereby allowing the other nodes on the loop to continue without disruption.
Arbitrated loop hubs may provide from 1 to 16 hub ports, with accommodation for more devices accomplished by cascading hubs together. A cascade is built by simply connecting a hub port of one hub to a hub port on the other, preferably with fibre optic cabling. In this way the total loop circumference is extended through additional hubs until the desired port count is reached.
However, such hubs have the disadvantage that the sequence of ports in the FC-AL through which a signal is transmitted is fixed by the internal wiring of the hub. Further such systems only allow the connection of one port to another thereby acting to include devices in the FC-AL.
Such hubs do not normally allow for branching type connections that would enable a device to sample information from the FC-AL without intervening in its activity. Also the activity of bypassing a loop in the hub introduces a delay into the loop traffic.